1. Field of the Invention
This invention relates generally to formation fluid testing and collection apparatus and more particularly to a closed-loop system for in situ determining the type and condition of formation fluids in a wellbore and for collecting downhole formation fluid samples under the original formation conditions.
2. Description of the Related Art
In the oil and gas industry, wireline formation testing tools have been used for monitoring formation pressures, obtaining formation fluid samples and for predicting reservoir performance. Such formation testing tools typically contain an elongated body having an inflatable packer that is sealingly urged against the zone of interest in the wellbore to collect formation fluid samples in storage chambers placed in the tool.
Various types of drilling fluids are used to facilitate the drilling process and to maintain a desired hydrostatic pressure in the wellbore. These drilling fluids penetrate into or invade the formations for varying radial depths (referred to generally as the invaded zone) depending upon the type the formation and drilling fluid used. Any initial fluid collected by the formation testing tools must first be analyzed to determine when the formation fluid being withdrawn is substantially free of mud filtrates and, thus, to collect only the uncontaminated fluid. Additionally, it is desirable to collect the formation fluids for further analysis in the same condition they exist in the formation. This typically requires that the fluid drawdown pressure be maintained above the bubble point of the fluids. The formation testing tools have utilized various sensors and in situ techniques to determine when the formation fluids being withdrawn are substantially free of mud filtrates and to maintain the drawdown pressure above the bubble point so as to collect clean fluids under the original formation conditions.
Resistivity measurements, downhole pressure and temperature measurements, and optical analysis of the formation fluids have been used to identify the type of formation fluid, i.e., to differentiate between oil, water and gas present in the formation fluid and to determine the bubble point pressure of the fluids. The information obtained from one or more pressure sensors and temperature sensors, resistivity measurements and optical analysis is utilized to control the drawdown rate so as to maintain the drawdown pressure above the bubble point and to determine when to collect the fluid samples downhole.
One prior art tool contains a resistivity measuring device and a temperature sensor as part of a probe assembly to monitor the characteristics of the formation fluid. An additional module containing optical fluid analyzer utilizes near-infrared spectroscopy absorption and reflection to differentiate between oil, water and gas. The fluid from the formation is discharged into the wellbore until the fluid flowing through the flow line is determined to be substantially free from contaminants. The fluid drawrate from the formation is controlled to maintain the drawdown pressure remains above the bubble point.
The interpretation of the flow line resistivity is difficult and often inaccurate. Interpretation of the resistivity must take fluid dynamics into consideration. The resistivity measured is that of the continuous phase of the fluid in the flow line. A water/hydrocarbon mixture with the water as the continuous phase has a low resistivity that increases due to tortuosity as the percentage of hydrocarbon increases. A water-hydrocarbon mixture with hydrocarbon as the continuous phase can has a high resistivity even if the water volume is large. Flow of alternating slugs of hydrocarbon and water produces noisy resistivity recording. This effect is more evident when gas is present.
The optical analyzer provides more accurate results, but is quite expensive and requires the use of sophisticated electronics downhole, which must operate at very high temperatures.
Thus, a need exists to provide a formation fluid retrieval and collection system that is relatively simple, less expensive than the current state-of-the-art systems and relatively accurate in differentiating between the various types of fluid conditions to ensure that substantially uncontaminated formation fluid samples are collected and that the drawdown pressure is maintained above the bubble point of the formation fluid during the fluid collection process.
The present invention addresses the above-noted deficiencies and provides a relatively simple closed loop system for collecting one or more formation fluid samples under original formation conditions.